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Show NOx EMISSIONS FOR LEAN-PREMIXED COMBUSTION Abstract Philip C. Malte, Robert C. Steele, David G. Nicol, Jon H. Tonouchi and Andrew C. Jarrett Combustion Laboratories Department of Mechanical Engineering, FU-10 University of Washington Seattle, Washington 98195 Lean-premixed combustion is a method by which the NOx emissions of practical gasflred equipment can be reduced to low levels. In this paper, the emission of NOx from lean-premixed combustion is examined through work conducted at the University of Washington with laboratory high intensity combustion reactors and computer models, and through a survey of the work published recently by investigators associated with the gas turbine engine manufacturers and the natural gas industry. The focus of the paper is on low emission gas turbine engines, though the fundamental results can be applied also to other gas-flred equipment. The amount of NOx formed in lean-premixed combustion is found to depend on the flame temperature, and on the degree to which the fuel and air are premixed. All cases examined show that the NOx increases exponentially with flame temperature, per an activation energy which lies between about 32 and 59kcal/gmol for most of the cases examined. This activation energy is substantially less than that for thermal NOx. Other variables affecting the NOx emission are the combustor pressure level, and the residence times of the flame zone and post-flame zone. Under some conditions, the inlet air temperature also is seen to affect the NOx emission. Trends in the NOx emissions as a function of these variables are provided and discussed. Practical levels of NOx emission are given. 1. INTRODUCTION Much of the electrical generating capacity being added worldwide today is based on the gas-fired gas turbine engine as the prime mover in co-generation (i.e., combined heat and power) and combined cycle systems. Lean-premixed combustion is becoming the standard method of combustion in these engines. Gas-fired gas turbine engines equipped with this combustion technology, which has undergone research and development since the mid 1970s, have NOx exhaust emissions in the 10 to 30 ppmvd (15% 02) range. Many of the new engines carry a guaranteed NOx emission of 25 ppmvd (15% 02). Heavy-duty gas turbine engines with lean-premixed combustion became commercially available in the late 1980s and early 1990s. For the most part, these were large engines, of size greater than 50MW, though today heavy-duty engines in the 20MW range are available with lean-premixed combustion. Lean-premixed combustion for aeroderivative gas turbine engines has taken longer to evolve, though this technology is now reaching and nearing commercial availability. Gas turbine engines for mechanical drive/gas pipeline applications are also becoming available with lean-premixed combustors. These engines currently carry an NOx guarantee of about 40 ppmvd (15% 02)· |
